1. Field of the Invention
The present invention relates to a digital data reproducing apparatus suitable for use in reproducing digital data recorded on a magnetic tape according to, for example, a Bi-phase Mark modulation or an 8-10 block code modulation.
2. Description of the Prior Art
It has been the practice, in a video tape recorder (VTR) of an 8-mm video system, for example, to PCM encode an audio signal and time compress the encoded signal. The resultant signal is then modulated according to a Bi-phase Mark modulation to produce an audio signal for recording along an audio track formed in connection with a video track, for example, as in U.S. Pat. No. 4,551,771.
The Bi-phase Mark modulation is a modulating method for digital data to be recorded on a magnetic tape, in which carrier signals S.sub.1 and S.sub.2 of frequencies f.sub.1 (=2.9 MHz) and f.sub.2 (5.8 MHz), for example, as shown in FIG. 1(A), are provided with a phase relationship such that their zero-cross points coincide with each other. For example, the carrier signal S.sub.1 of the lower frequency f.sub.1 is recorded on a magnetic tape when, for example, a PCM audio signal is logical "L", while the carrier signal S.sub.2 of the higher frequency F.sub.2 is recorded when the PCM audio signal is logical "H".
When an audio signal recorded along an audio track is to be reproduced, zero-cross points of the picked up carrier signal S.sub.1 or S.sub.2 are detected as shown in FIG. 1(B), and then, pulse outputs PLL.sub.0 are generated at a sampling pulse oscillator constituted by, for example, a PLL (phase locked loop), such pulse outputs PLL.sub.0 having a 45.degree. phase angle with respect to the carrier signal S.sub.1 of the lower frequency f.sub.1, and having a 90.degree. phase angle with respect to the carrier signal S.sub.2 of the higher frequency f.sub.2.
Among the pulse outputs PLL.sub.0, those appearing with phase angles of 45.degree., 135.degree., 225.degree., 315.degree. of the carrier signal S.sub.1 are used as sampling pulses P.sub.S (FIG. 1(C). If positive values are obtained at every two sampling pulses, the reproduced data is determined to be of a logical "L".
The pulse outputs PLL.sub.0, with phase angles of 90.degree., 270.degree. in respect to the carrier signal S.sub.2 are used as sampling pulses P.sub.S (FIG. 1(C). If a positive value is obtained at alternate ones of these samplings, the reproduced data is determined to be of a logical "H".
A prior art digital data reproducing apparatus 1 for reproducing the audio data recorded along the audio track in the Bi-phase Mark modulation is arranged, as shown in FIG. 2, so that a recovered clock signal CK.sub.1 is extracted from a reproduced carrier signal S.sub.PB picked up by a magnetic head 2 from a magnetic tape 3, the signal CK.sub.1 having a repetition frequency included in the signal S.sub.PB, and the reproduced carrier signal S.sub.PB (DT.sub.PB1) is demodulated on the basis of the recovered clock signal CK.sub.1.
More specifically, the reproduced carrier signal S.sub.PB obtained at the time when the magnetic head 2 scans the video track on magnetic tape 3 is supplied through conventional circuits, for example, a switching circuit (not shown) to a specific video signal processing circuit (not shown), and, and on the other hand, the reproduced carrier signal S.sub.PB obtained at the time when the magnetic head 2 scans the audio track on a magnetic tape 3 is supplied through a reproducing amplifier 4 to an equalizer circuit 5, at which a specific equalizing process is performed, and the equalized signal is supplied to the inverting input terminal of a comparator 6 constituted by an operational amplifier circuit.
The noninverting input terminal of the comparator 6 is connected to a power source 6A providing a predetermined reference voltage V.sub.REF, and thus, the comparator 6 compares the reproduced carrier signal S.sub.PB with the reference voltage V.sub.REF and supplies the reproduced digital signal DT.sub.PB obtained as the result of the comparison to an input terminal D of a synchronizing circuit 7 in the form of a D-type flip-flop structure. The reproduced digital signal DT.sub.PB is also supplied to a clock recovery circuit 8 including a phase-locked loop (PLL) circuit.
The clock recovery circuit 8 functions to compare the phase of a reference signal with a predetermined frequency obtained from a voltage-controlled oscillator (VCO) incorporated therein with the phase of the clock component having a predetermined repetition frequency included in the input reproduced digital signal DT.sub.PB and thereby obtains the recovered clock signal CK.sub.1 precisely phase-locked to the clock component. The recovered clock signal CK.sub.1 is supplied to the clock input terminal C of the synchronizing circuit 7 and also to a demodulating process circuit 9.
In this way, the synchronizing circuit 7 synchronizes the reproduced digital signal DT.sub.PB with the recovered clock signal CK.sub.1 and delivers the synchronized digital signal DT.sub.PB1 to the subsequent demodulating process circuit 9. Therefore, the demodulating process circuit 9 demodulates the incoming reproduced digital signal DT.sub.PB1 on the basis of the recovered clock signal CK.sub.1.
Recently a system has been proposed for recording audio signals in an 8-mm VTR other than with the above described Bi-phase Mark modulation (hereinafter referred to as the "first record modulation mode"). Such system will be used, for example, in a rotary head digital audio tape recorder (R-DAT). According to this system, as disclosed in U.S. Pat. Nos. 4,617,552 and 4,577,180, when PCM encoding an audio signal, the audio signal is encoded in a much higher recording density than that in the first record modulation mode and the coded signal is then modulated by 8-10 block code modulation (hereinafter to be referred to as a "second record modulation mode" prior to being recorded along the audio track.
In practice, however, the frequency band widths of the recorded signals are quite different for the first and second record modulation modes. Further, the frequencies of the sampling clock are greatly different. For example, the sampling clock frequency in the first record modulation mode is 11.6 MHz while that in the second record modulation mode is 14.8 MHz. Thus, the Nyquist conditions in the electromagnetic conversion systems and the noise spectrum distributions are substantially different for the first and second record modulation modes.
For these reasons, in order for the data recorded in the first or the second record modulation mode to be reproduced from a magnetic tape, it is preferred that the digital reproducing apparatus be provided with first and second equalizer circuits having equalizing characteristics adapted for the respective record modulation modes. Further, first and second demodulating process circuits adapted for their respective record modulation modes, are provided, and the mode of the record modulation used for making the record on the magnetic tape 3 is detected in some way or other, whereupon, the first or the second equalizer circuit and the first or the second demodulating process circuit are selectively used according to the result of the aforesaid detection.